Method of forming an article

ABSTRACT

Methods of forming an article from a molten plastic composition comprising a polymer and a blowing agent. The methods include injecting the molten plastic composition into a mould, allowing the plastic composition to form a first solid skin adjacent to an in contact with a first cavity-forming surface of the mould and a second solid skin adjacent to and in contact with a second cavity-forming surface of the mould, and then opening the mould before the molten plastic composition between the first and second solid skins in at least one portion of the circumference of a region of the mould cavity defining an annular cross-section of the cavity has solidified.

FIELD OF THE INVENTION

The present invention relates to the formation of foamed plasticarticles, e.g. cups or containers for food.

BACKGROUND

In the packaging industry, a commonly-used type of disposable cup (forexample, take-away coffee cups) is a paper cup with an inner lining of aplastic material, e.g. low-density polyethylene (LDPE). As these cupsare made of two different materials which can be difficult and/or costlyto separate, their recycling can prove challenging. Furthermore, asthere is a seam down one side of the cup where the paper material isjoined together, liquid may leak from the area of the join at the rim ofthe cup when the cup is tilted for consuming the beverage within(particularly when the cup is used in connection with a lid having amouthpiece through which the beverage passes for consumption).

Efforts have been made in the industry to provide disposablepolypropylene cups. For example, US-A-2014/0166738 discloses a blankmade of a cellular polymeric material such as polypropylene, which blankcan then be formed into a cup. However, a seam is still present in thecup of US-A-2014/0166738, and the final cup must be made by way offolding the blank and sealing the various edges thereof together.Furthermore, in conventional injection moulding a thick wall is requiredto create some thermal insulation in a cup. Typically, a cellularstructure provides the thermal insulation, and a blowing agent is addedto the thermoplastic polymer to create a foamed structure to furtherimprove the thermal insulation, and to reduce the density of the foamedwall. However, for a given wall thickness the maximum density reductionachievable by foaming is 30 wt % based on the weight of thethermoplastic polymer in the wall.

SUMMARY OF THE INVENTION

The present invention aims at least partially to overcome the problemswith existing disposable containers. In particular, there is a need inthe art for containers which are easily recyclable. It is also desirableto reduce the amount of material used to make each container, whilststill providing a container which has good thermal insulation propertiesand the required level of stiffness.

In a first aspect, the present invention provides a method of forming anarticle, the method comprising:

(a) providing a mould having an outer part and an inner part, the outerpart having a first cavity-forming surface and the inner part having asecond cavity-forming surface;

(b) closing the mould thereby defining a cavity between the first andsecond cavity-forming surfaces wherein, at least one region of thecavity defining an annular cross-section of the cavity between outer andinner circumferential edges of the cavity, the first cavity-formingsurface is such that a distance between the first cavity-forming surfaceand the second cavity-forming surface alternates between a firstdistance and a second distance around at least a portion of thecircumference of the region, the first distance being greater than thesecond distance;

(c) injecting a molten plastic composition comprising a polymer and ablowing agent into the cavity;

(d) allowing the plastic composition to form, in the said at least oneregion, a first solid skin adjacent to and in contact with the firstcavity-forming surface and having a first circumferential length, and asecond solid skin adjacent to and in contact with the secondcavity-forming surface and having a second circumferential length,wherein the plastic composition between the first and second solid skinsremains molten, to form, respectively where the distance between thefirst cavity-forming surface and the second cavity-forming surfacealternates between the first distance and second distance, alternatingfirst and second thicknesses of the plastic composition, each first andsecond thickness comprising the first and second skins with moltenplastic composition therebetween;

(e) opening the mould before the plastic composition between the firstand second solid skins has solidified, thus allowing the plasticcomposition between the first and second solid skins, and the moltenplastic composition at the alternating first and second thicknesses ofthe plastic composition, to expand by foaming, wherein said openingcomprises removing the outer part of the mould so that the first solidskin is no longer in contact with the first cavity-forming surface,while maintaining the second solid skin in contact with the secondcavity-forming surface.

The second solid skin may be maintained in contact with the secondcavity-forming surface until expansion of the plastic compositionbetween the first and second solid skins is complete.

The first and second distances, and correspondingly the first and secondthicknesses, are configured to provide an alternating series of peaksand troughs, or ribs and valleys, around the circumference of theinjection moulded composition. These values can be constant values oralternatively both the first and second distances and thicknesses canindependently vary around the circumference. In some embodiments of thepresent invention, the first and second distances and thicknesses areconstant values around the circumference. In other embodiments of thepresent invention, the second distance/thickness is a constant valuearound the circumference and the first distance/thickness varies aroundthe circumference. In other embodiments of the present invention, thefirst distance/thickness is a constant value around the circumferenceand the second distance/thickness varies around the circumference. Inother embodiments of the present invention, the seconddistance/thickness is a constant value around the circumference and thefirst distance/thickness varies in alternation between two differentfirst values around the circumference. In other embodiments of thepresent invention, the first and second distances/thicknesses bothindependently vary around the circumference.

In a second aspect, the present invention provides a hollow article foruse as a beverage cup, or as a container, the hollow article having anannular sidewall and a base wall defining a central hollow cavity, a toprim of the sidewall and a bottom end of the sidewall, wherein theannular sidewall comprises a plastics material composed of a sandwichstructure of inner and outer skins and an expanded cellular foam layertherebetween, wherein the expanded cellular foam layer comprises anannular array of reinforcing areas extending longitudinally along thesidewall in a direction between the top rim and the bottom end, thereinforcing areas being mutually separated by an annular array of spacerregions extending longitudinally along the sidewall in a directionbetween the top rim and the bottom end to provide alternatingreinforcing areas and spacer regions around the annular sidewall,wherein the reinforcing areas comprise expanded cellular foam of a firstdensity and the spacer regions comprise expanded cellular foam of asecond density, wherein the first density is higher than the seconddensity.

Preferred features of each aspect are defined in the respectivedependent claims.

Advantages of present invention are that the container formed by themethod thereof is easily recyclable and reusable, has no join in thematerial through which leakage of a liquid contained thereon couldoccur, has the required level of stiffness, and also possesses goodthermal insulation properties. Furthermore, the present invention allowsfor less material to be used for the manufacture of each container(hence reducing manufacturing costs).

Using the present invention, it is possible to achieve a densityreduction of 200% between the density of the thermoplastic resin in itsunfoamed state and the density of the thermoplastic material in the cup,which includes regions of expanded cellular foam and unfoamed regions.

The present invention is at least partly predicated on the finding bythe present inventor that the provision of a first cavity formingsurface which is such that a distance between the first cavity-formingsurface and the second cavity-forming surface alternates between a firstdistance and a second distance (the first distance being greater thanthe second distance) allows for a reduction in the amount of materialused to form the article, and also thus in a reduction in the weight ofthe article. In particular, in the first aspect, the present inventorhas discovered that the first solid skin (as formed prior to opening themould) can be “blown out” by foaming of the molten plastic compositionupon opening the mould (the mould being opened prior to solidificationof the molten plastic composition between the first and second skins). Adistance between the first and second solid skins of the finishedarticle may be substantially constant around the circumference of thefinished article. Thus, the method of the present invention uses lessmaterial but provides a finished product which is similar in appearanceto the product obtained when the distance between the firstcavity-forming surface and the second cavity-forming surface in themould remains constant rather than alternating between a first andsecond distance.

In the second aspect, the present inventor has found that, when forminga finished container in which some areas comprise expanded plasticcomposition and some areas comprise non-expanded plastic composition,the provision of the first cavity-forming surface which is such that adistance between the first cavity-forming surface and the secondcavity-forming surface alternates between a first distance and a seconddistance around at least one first portion of the circumference of theregion (the first portion being the area which expands upon opening ofthe mould), makes it possible to reduce that amount of material used toform the article, and also hence reduce the weight of the article. Inparticular, the first solid skin in the first portion (as formed priorto opening the mould) can be “blown out” by foaming of the moltenplastic composition between the first and second skins in the firstportion, so that the first skin is convex in the first portions in thefinished article. Thus, the method of the present invention uses lessmaterial but provides a finished product which is similar in appearanceto the product obtained when the distance between the firstcavity-forming surface and the second cavity-forming surface in themould remains constant rather than alternating between a first andsecond distance.

The present inventor has also found that the articles produced by themethod of the present invention also have the level of stiffnessrequired for such articles, despite the reduction in the amount ofmaterial used therein. It has been found that as the cellular foamedplastic composition, typically a thermoplastic such as a polyolefin,typically polypropylene, cools slowly, due to its thermal insulationqualities, the crystallinity of the plastic composition can increase,which in turn can increase the rigidity of the cellular foamed plasticcomposition. The expansion of the molten plastic composition between thefirst and second skins by foaming also provides the articles with goodthermal insulation properties.

Also, since the entire container may be made of a single layer ofrecyclable material (i.e. no layers of different materials which need tobe separated), the container is easier to recycle than the commonly usedplastic-lined paper cups. Where a label or outer coating (film sheet orfilm sleeve) is added to the container during the manufacturing method,this may also be of the same material from which the container itself isformed. For example, the polymer and the film sheet may both be formedof polypropylene.

Furthermore, as the articles are injection moulded in the methods of thepresent invention, there is no join present in the article through whichleakage of a liquid contained therein could occur.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described by way ofexample only with reference to the accompanying drawings, in which:

FIG. 1 is a cross-section of an article illustrating the appearance ofthe article at different stages in the method of the first aspect of thepresent invention;

FIG. 2A shows a cross-section through a part of the article at the endof step (d) of one embodiment of the method of the first aspect of thepresent invention, when the article is still within the mould;

FIG. 2B shows the cross-section as shown in FIG. 2A, illustrating thedensity of the molten plastic composition at different areas within thenon-expanded article, when the article is still within the mould;

FIG. 3 shows a cross-section through the fully expanded article afteropening the mould in step (e);

FIGS. 4A and 4B show the density of the foam at different points in theexpansion process to produce the article illustrated in FIG. 3;

FIG. 5 illustrates the expansion of the molten plastic composition inthe article shown in FIG. 3;

FIG. 6 shows a cross-section through a part of the article at the end ofstep (d) of one embodiment of the method of the second aspect of thepresent invention, wherein the left hand side of the Figure shows thearticle still within the mould, and the right hand side of the Figureshows the article when it has expanded and been removed from the mould;

FIG. 7 shows two first portions 116 and three second portions 118 of thearticle at the end of step (d) of the second aspect of the presentinvention, when the article is still in the mould;

FIG. 8 shows a cross-section through a part of the article at the end ofstep (d) of one embodiment of the method of the second aspect of thepresent invention, when the article is still within the mould;

FIG. 9 is a side view of an injection moulded preform for a cup formedaccording to a method of a further embodiment of the present invention;

FIG. 10 is a cross-section though a sidewall of the injection mouldedpreform of FIG. 9;

FIG. 11 is a bottom view of the base of the injection moulded preform ofFIG. 9;

FIG. 12 is a side view of a cup formed by expansion of the injectionmoulded preform of FIG. 9;

FIG. 13 is a cross-section though a sidewall of the cup of FIG. 12;

FIG. 14 is a top view through a transverse cross-section of the cup ofFIG. 12.

DETAILED DESCRIPTION

Referring to FIG. 1, there is shown a cross-section of an article 2 (forexample, a coffee cup) illustrating the appearance of the article atdifferent stages in the method of the first aspect of the presentinvention. In particular, the left-hand side of FIG. 1 shows the firstsolid skin 4, the second solid skin 6 and the molten plastic composition8 between the first and second solid skins 4, 6 at the end of step (d)of the method of the first aspect of the invention, when the article isstill within the mould. FIG. 1 illustrates an embodiment where the firstcavity-forming surface of the outer part 12 of the mould comprisescorrugations, the corrugations in the first cavity-forming surfacehaving peaks and troughs in the form of a sinusoidal wave (noting that atrough in the first cavity-forming surface results in a correspondingpeak being formed in the article moulded therein, and vice-versa). Thesinusoidal wave may have a variety of different configurations withrespect to the shape and morphology of the peaks and troughs.

The corrugations may for example be U- or V-shaped and/or, for any shapeof the peaks and troughs, the corrugations may have a repeatable, orregular, sequence of peaks and troughs or a non-repeatable, or random,sequence of peaks and troughs.

Typically, the peaks and troughs have some degree of curvature, withlarge radii of curvature. The inner part 14 of the mould, which is amould core, is also shown. In this embodiment, the first distance D₁ andthe second distance D₂ (both of which are measured perpendicular to atangent to the second cavity-forming surface) each remain constantaround the circumference of the region of the mould cavity which definesan annular cross-section of the cavity between outer and innercircumferential edges of the cavity. The first distance D₁ is thedistance between the second cavity forming surface and a lowest point ofa trough in the first cavity-forming surface and the second distance D₂is the distance between the second cavity-forming surface and a highestpoint of a peak in the first cavity forming surface.

As explained hereinabove, the first and second distances D₁, D₂ areconfigured to provide an alternating series of peaks and troughs, orribs and valleys, around the circumference of the injection mouldedcomposition. These values can be constant values or alternatively boththe first and second distances D₁, D₂ can independently vary around thecircumference. In some embodiments of the present invention, the firstand second distances D₁, D₂ are constant values around thecircumference. In other embodiments of the present invention, the seconddistance D₂ is a constant value around the circumference and the firstdistance D₁ varies around the circumference. In other embodiments of thepresent invention, the first distance D₁ is a constant value around thecircumference and the second distance D₂ varies around thecircumference. In other embodiments of the present invention, the seconddistance D₂ is a constant value around the circumference and the firstdistance D₁ varies in alternation between two different first valuesaround the circumference. In other embodiments of the present invention,the first and second distances D₁, D₂ both independently vary around thecircumference.

The right-hand side of FIG. 1 shows the first solid skin 4, the secondsolid skin 6, and the plastic composition between the first and secondsolid skins 4, 6, which plastic composition 10 has expanded by foamingand solidified. In the right-hand side of FIG. 1, the outer part 12 ofthe mould has been removed from the article and the article remains onthe inner part 14 or core. As can be seen, the expansion has resulted inthe areas formed in the first solid skin 4 where the distance is thesecond distance D₂, being “blown out” so that the distance between thefirst and second solid skins 4, 6 of the finished article issubstantially constant (e.g. varies by up to plus or minus 2% ascompared to the average distance between the first and second skins)around the circumference of the article.

At the end of step (d) in the methods of both the first and secondaspects of the present invention, the areas where the distance betweenthe first and second cavity-forming surfaces is the first distance D₁(referred to henceforth as the “ribs”) will store latent heat, and willbe hotter than those areas where the distance between the first andsecond cavity-forming surfaces is the second distance D₂ (referred tohenceforth as the “valleys”). The ribs act as flow promoters during thefilling phase of the injection moulding process, i.e. step (b) in themethods of the first and second aspects of the present invention. As theinjected material takes the path of least resistance, the ribs will bethe hottest parts of the article. This storage of latent heat allows thefirst solid skin 4 to be deformed upon opening of the mould by thepressure of the gas released from the blowing agent. This pressure actsto pull the first solid skin 4 away from the second solid skin 6. Thisoccurs at the ribs first (i.e. the hottest part of the article), and thepressure of the first skin 4 being pulled away at the area of the angledparts of these ribs (either side of the highest point D₁ of the rib)then acts to pull the first skin 4 away from the second skin 6 at thearea of the adjacent valleys. This effect (termed the “wedge effect”) isdiscussed in more detail, below.

FIG. 2A shows a cross-section through a part of the article at the endof step (d) of one embodiment of the method of the first aspect of thepresent invention, when the article is still within the mould. As inFIG. 1, the first solid skin 4, the second solid skin 6 and the moltenplastic composition 8 between the first and second solid skins 4, 6 areshown. In this embodiment, the first cavity forming surface of the outerpart 12 of the mould again comprises corrugations, the corrugationshaving peaks and troughs. However, in contrast to the embodiment shownin FIG. 1, the first distance D₁ varies around the circumference of theregion of the mould cavity which defines an annular cross-section of thecavity between outer and inner circumferential edges of the cavity. Inparticular, the first distance D₁ is at a maximum value D_(1(max)) atevery fourth trough of the mould cavity, and the first distance reachesa minimum value D_(1(min)) at each trough which is midway between twotroughs at which the first distance is at the maximum value. Again, itis noted that a trough in the first cavity-forming surface of the mouldresults in a corresponding peak being formed in the article mouldedtherein, and vice-versa. In the embodiment shown in FIG. 2A, the seconddistance D₂ remains constant around the circumference of the region ofthe mould cavity.

FIG. 2B shows the cross-section as shown in FIG. 2A, illustrating thedensity of the molten plastic composition 8 at different areas withinthe non-expanded article. As can be seen from this Figure, the densityof the molten plastic composition 8 (prior to its expansion by foamingbetween the first and second solid skins) varies from higher density inthose areas where the distance between the first and secondcavity-forming surfaces of the mould is the second distance D₂, to lowerdensity in those areas where the distance between the first and secondcavity-forming surfaces is the first distance D₁—the lower density areas20 are represented by open cells 22 indicating the presence of expandedcellular foam 24 in the outer parts 28 of the ribs 30. In the first andsecond solid skins 4, 6 themselves, and in the small thickness regions32, for example the valleys 34, other than the outer parts 28 of theribs 30, the density of the molten plastic composition 8 is very high,with minimal or no foaming.

In the cross-section shown in FIG. 2B, while the article is in themould, the rib where the first distance is D_(1(min)) will be startingto cool and solidify at the point of opening the mould. The ribs wherethe first distance is denoted as “D_(1(int))” (D_(1(int)) being a valuebetween D_(1(min)) and D_(1(max))), will cool faster than the ribs wherethe first distance is D_(1(max)). The ribs where the first distance isD_(1(max)) will thus retain the most heat, and will be the hottest areasof the article upon opening of the mould. Similarly to the discussionabove, this allows the first solid skin 4 to be deformed at the ribswith first distance D_(1(max)) upon opening of the mould, by thepressure of the gas released from the blowing agent. This pressure actson the whole of the outer solid skin 4, the inner solid skin 6 beingsupported by the mould core 14.

The pressure acts to push the first, outer, solid skin 4 away from thesecond, inner, solid skin 6. The pushing apart of the first and secondskins 4, 6 is at the area of the valleys which have retained a highconcentration of blowing agent. The first, outer, solid skin 4 may befurther pushed away from the second, inner, solid skin 6 in the vicinityof the ribs where there is sufficient pressure from the remainingblowing agent which was not expanded within the injection mould.

FIG. 3 shows the fully-expanded article in which the first, outer solidskin 4″ has been fully pushed away from the second, inner, solid skin 6across the area of the both the ribs, which may have first distanceD_(1(max)),D_(1(min)), and D_(1(int)), and the valleys between theseribs having the second distance D2, which may be the same or variablefor the valleys.

The density of the expanded cellular foam is lower than the density ofthe molten unexpanded plastic composition. As shown in FIG. 4A, in theribs 30 of the injection moulded article, some cellular voids 22 areformed by expansion of the blowing agent in the large thickness regions,whereas in the valleys 34 of the injection moulded article there issubstantially no expansion of the blowing agent in the small thicknessregions. As shown in FIG. 4B, after opening the mould, the gas pressureremains evenly distributed for several seconds, maintaining a forceagainst the outside skin 4″, and therefore in the ribs 30 of theinjection moulded article, the cellular voids 22 tend to grow and/orcoalesce by further expansion of the residual blowing agent in the largethickness regions, whereas in the valleys 34 of the injection mouldedarticle the expansion of the blowing agent is initiated in the smallthickness regions 32.

The result is that, as shown in FIG. 4B, in the final expanded article40 there are circumferentially alternating low density regions 42 andhigh density regions 44. The low density regions 42 correspond to thelocation of the ribs 30 and are associated with relatively largedimension cellular voids 46, whereas the high density regions 44correspond to the location of the valleys 34 and are associated withrelatively small dimension cellular voids 48. The average cell size issmall, and the cell size is substantially uniform, in the high densityregions 44, to provide a substantially homogeneous foam structure,whereas the average cell size is larger, and the cell size is lessuniform, in the low density regions 42, to provide a less homogeneousfoam structure. In FIG. 4B, it can be seen that the foam density ishigher at the skins, and lower in the centre of the annularcross-section. FIG. 4B, also shows the alternating high and low densityfoam that relate to the low density in the ribs and the high density inthe valley areas.

FIG. 5 illustrates that when the ribs have different heights, afterinjection moulding the thicker ribs 50 may have a larger number ofcellular voids 52 and/or larger cellular voids 52 than the cellularvoids 56 in the thinner ribs 54. In the vicinity of the valleys 58 thereis a high concentration of blowing agent, for example CO₂ in solution.On opening the mould, thereby to leave the injection-moulded article onthe core, the outer skin 4 above the valleys 58 expands rapidly as aresult of the high blowing agent concentration, for example causing CO₂to come out of solution and to form a gas. The tension holding the firstand second skins 4, 6 together then decreases as the foam densityreduces, as a result of the formation of cellular voids, and as pressureis exerted on the skins from gas released from the blowing agent in themolten plastic composition. The two skins are then pushed apart by thepressure exerted by the gas, and this pressure—along with the lowerdensity of plastic composition at area D, which reduces the ability ofthe plastic composition at this point to restrain the first and secondskins—then allows the first and second skins to also be pushed apart atthe valley (area D) and form the fully-expanded article.

In a second aspect of the present invention, the first cavity-formingsurface is such that a distance between the first cavity-forming surfaceand the second cavity-forming surface alternates between a firstdistance and a second distance around at least one first portion of thecircumference of the region, the first distance being greater than thesecond distance; and each first portion is disposed between a pair ofsecond portions of the circumference of the region, in which secondportions a distance between the first cavity-forming surface and thesecond cavity-forming surface is a third distance which is less than thesecond distance. In step (d), the plastic composition between the firstand second solid skins is allowed to solidify in said second portions,while the plastic composition between the first and second solid skinsremains molten in said at least one first portion. In step (e), themould is then opened and the plastic composition between the first andsecond solid skins in said at least one first portion is then allowed toexpand by foaming The foaming is a result of the pressure drop onopening the mould, and causing the blowing agent, for example CO₂, tocome out of solution creating the expansion of the gas provided by theblowing agent. As discussed above, opening of the mould comprisesremoving the outer part of the mould so that the first solid skin is nolonger in contact with the first cavity-forming surface, whilemaintaining the second solid skin in contact with the secondcavity-forming surface.

FIG. 6 shows a cross-section through a part of the article 102 (forexample, a coffee cup) illustrating the appearance of the article atdifferent stages in the method of the second aspect of the presentinvention. The left-hand side of FIG. 6 shows the first solid skin 104,the second solid skin 106, the molten plastic composition 108 betweenthe first and second solid skins 104, 106 in the first portions 116 ofthe circumference of the region, and the solidified 120 plasticcomposition between the first and second solid skins 104, 106, in thesecond portions 118 of the circumference of the region, at the end ofstep (d) of the method of the first aspect of the invention, when thearticle is still within the mould. FIG. 6 illustrates an embodimentwhere the first cavity forming surface of the outer part 112 of themould comprises corrugations, the corrugations having peaks and troughsin the form of a sinusoidal wave. The inner part 114 of the mould isalso shown. In this embodiment, the first distance D₁₀₁ and the seconddistance D₁₀₂ remain constant around the circumference of the region ofthe mould cavity which defines an annular cross-section of the cavitybetween outer and inner circumferential edges of the cavity. In thesecond portions 118, the third distance is shown as D₁₀₃. The distancesbetween the first cavity-forming surface and the second cavity-formingsurface are measured perpendicular to a tangent to the secondcavity-forming surface.

The right-hand side of FIG. 6 shows the first solid skin 104, the secondsolid skin 106, and the plastic composition between the first and secondsolid skins 104, 106 in the first portions, which plastic composition110 has expanded by foaming and solidified. In the right-hand side ofFIG. 6, the article has been removed from the mould. As can be seen, theexpansion has resulted in the valleys formed in the first solid skin 104being “blown out” so that the first solid skin 104 is convex in thefirst portions 116 of the finished article.

At the end of step (d) in the method of the second aspect of the presentinvention, the areas where the distance between the first and secondcavity-forming surfaces is the first distance D₁ (referred to henceforthas the “ribs”) will store latent heat, and will be hotter than thoseareas where the distance between the first and second cavity-formingsurfaces is the second distance D₂ (referred to henceforth as the“valleys”), as discussed above with reference to the first aspect of thepresent invention.

FIG. 7 shows two first portions 116 and three second portions 118 of thearticle at the end of step (d) of the second aspect of the presentinvention, when the article is still in the mould. The distances D₁₀₁,D₁₀₂ and D₁₀₃ can be seen more clearly in this Figure. In thisembodiment, the first distance D₁₀₁ varies around the first portion 116of the circumference of the region.

FIG. 8 shows a cross-section through a part of the article at the end ofstep (d) of one embodiment of the method of the second aspect of thepresent invention, when the article is still within the mould. As inFIG. 6, the first solid skin 104, the second solid skin 106 and themolten plastic composition 108 between the first and second solid skins104, 106 of the first portion are shown, along with the solidifiedplastic composition 120 between the first and second solid skins 104,106 of the second portions 118. In this embodiment, the first cavityforming surface of the outer part 112 of the mould again comprisescorrugations in the first portion, the corrugations having peaks andtroughs. However, in contrast to the embodiment shown in FIG. 6, thefirst distance D₁₀₁ varies around the first portion of the circumferenceof the region of the mould cavity which defines an annular cross-sectionof the cavity between outer and inner circumferential edges of thecavity. In particular, the first distance D₁₀₁ is at a maximum valueD_(101(max)) at the troughs of the first cavity-forming surface whichare adjacent to the second portions 118, and the first distance reachesa minimum value D_(101(min)) the pair of troughs which are midwaybetween the troughs of the first cavity-forming surface at which thefirst distance is at the maximum value D_(101(min)). Again, it is notedthat a trough in the first cavity-forming surface of the mould resultsin a corresponding peak being formed in the article moulded therein, andvice-versa. In the embodiment shown in FIG. 8, the second distance D₁₀₂remains constant around the circumference of the region of the mouldcavity.

The expansion mechanism in step (e) of the first aspect of theinvention, as discussed above regarding FIGS. 2A, 2B, 3, 4A, 4B and 5 isalso applicable to the expansion mechanism in step (e) of the secondaspect of the invention.

In both the first and second aspects of the present invention, the limitto the minimum thickness of the valleys (i e minimum value of D₂ whichstill allows for the first and second skins to be separated at thevalleys following opening of the mould) is controlled by the tensionbetween the first and second solid skins 4, 6 and the gas pressure inthe expanding foam structure that separates the solidified surfaceskins. The parameters that influence the tension are:

-   -   1. Temperature of the molten plastic composition    -   2. Percentage of blowing agent (which is typically an        endothermic blowing agent, but could be exothermic)/or        percentage and type of physical gas.    -   3. Speed of injection into the mould at step (b).    -   4. Injection pressure (higher pressure keeps more gas in        solution and reduces premature foaming during injection).    -   5. Cooling time.    -   6. Mould temperature.    -   7. Cell size in the foam.    -   8. Additives present in the molten plastic composition.

A non-limiting example of parameters which can be selected for each ofpoints 1 to 8, above, is given below. This example is purelyillustrative of a particular embodiment, and is not limiting on theinvention as a whole.

-   -   1. The melt temperature of polypropylene is 165° C. However,        chemical endothermic blowing agents generally need a higher        temperature to activate the reaction. Also, the higher the        temperature of the molten plastic composition, the smaller the        second distance D₂ can be. For a composition where the polymer        is polypropylene, melt temperatures of between 250° C. and        285° C. can be used in order to minimize the second distance D₂.        The same temperature range of 250° C. to 285° C. may also be        used in order to minimize the second distance D₂ when a physical        gas is used rather than a chemical blowing agent.    -   2. The blowing agent may be a chemical blowing agent or a        physical blowing agent or any mixture of chemical and/or        physical blowing agents. For example, the blowing agent may be a        mixture of chemical blowing agents, for example a combination of        endothermic and exothermic chemical blowing agents, or a        combination of chemical and physical blowing agents, for example        where the chemical blowing agent helps to nucleate a gas forming        the physical blowing agent. Alternatively, the blowing agent may        be a mixture of physical gases, for example a mixture of CO₂ and        N₂, optionally in further combination with a chemical blowing        agent. Chemical blowing agents for use in the present invention        are preferably 50 to 60 wt % active level agents at a        concentration of from 2 to 6 weight %, based on the weight of        the molten plastic composition, or a comparable quantity of        physical gas e.g. CO₂ or N₂ (a 60 wt % active level agent at a        concentration of 2 weight % would give 1.2 wt % gas in the        molten plastic composition, therefore the quantity of physical        gas could be from 1 to 2 wt %). A physical gas such as nitrogen        can easily and cheaply be added at any wt % within the range of        0.1 to 10 wt %), for example 6 wt %; higher concentrations of        the physical gas tend to progressively increases the forces of        the blowing agent outwardly against the skins.    -   3. Very fast injection speeds are preferred—for example, an        injection rate of 50 grams per second per cavity is needed to        ensure a filling time of less than 0.5 seconds; if the filling        time is longer than this it will allow the solidified skin        thickness to increase, thus reducing the thickness of the layer        of molten plastic composition between the first and second solid        skins 4, 6 at the end of step (d), and reducing the expansion        effects.    -   4. The cooling time within the mould before opening in step (e)        should also be minimised: however, this is limited by the time        it takes to reduce the locking force of the injection moulding        machine before opening the mould. Typically the time taken to        reduce the locking force is 0.2 to 0.5 seconds; unless other        special mechanisms are used, this parameter defines the minimum        time before the mould halves can start to move apart allowing        the first and second skins to start to move away from each        other.    -   5. The mould temperature affects the solid skin thickness. To        slow down the skin solidification speed for a polypropylene        cup-type product, the inner part of the mould is preferably at        40° C. to 70° C. Too high a temperature would cause the second        solid skin to deform, which may be undesirable. The outer part        of the mould is preferably at 50° C. to 120° C. in order to keep        the first skin soft enough to deform.    -   6. Cell size in the expanded foam is preferably maximized: a        fine cell structure has a greater melt strength preventing skin        separation. When the pressure of the molten plastic composition        reduces due to its flow into the expanded low pressure adjoining        area, the cell size increases. Larger cells have thinner skins        and therefore a lower melt strength. The cell size in the first        and second skins should be from 0 to 100 μm (micrometres), and        the cell size in the expanded composition between the skins will        be 50 to 250 μm or 100 to 500 μm or 250 to 1000 μm, or there        could be a void between the first and second skins.    -   7. Additives can be used to further influence the minimum value        of the second distance D₂. For example, chalk (calcium        carbonate) will hold heat for longer than polypropylene, and        thus its inclusion will help to slow down the skin        solidification. Mica, due to its sharp edged platelets, will        prevent bubbles from being formed, thereby breaking the adhesion        between the skins by creating a void.

As described above, typically the sinusoidal configuration for the peaksand troughs in the injection moulded intermediate article has smoothcurvature. In alternative configurations, the peaks and/or troughs mayhave shallow angles, for example forming a ‘V’ shape, and small radii ofcurvature; however such small radii would tend to resist being “pulled”out and leave vertical ridges in the wall of the cup.

In some embodiments, it may be desired to have a series of ridgesrunning vertically up the cup wall and this may avoid any stretching ofthe outer skin. This can be achieved by reducing the % of blowing agentand/or increasing the cooling time, which would tend to reduce theeffect of the blowing, and thereby achieve a partially blown cup, withthe valleys not fully blow out to the height of the peaks.

In some embodiments of the first and second aspects of the presentinvention, in step (e), the first circumferential length remainssubstantially constant as compared to the first circumferential lengthat the end of step (d). In some embodiments, the second circumferentiallength remains substantially constant as compared to the secondcircumferential length at the end of step (d). In some embodiments, boththe first and second circumferential lengths remain substantiallyconstant as compared to the first and second circumferential lengths atthe end of step (d). By “substantially constant” in this context, it ismeant that the particular circumferential length increases by up to 2%as compared to that circumferential length at the end of step (d).

In other embodiments of the first and second aspects, in step (e), thefirst circumferential length increases by up to 20%, up to 10%, or up to5% as compared to the first circumferential length at the end of step(d). In certain embodiments, the first circumferential length increasesby 12 to 20%, optionally 8 to 15%, as compared to the firstcircumferential length at the end of step (d); alternatively by 3 to 10%or 1 to 5% as compared to the first circumferential length at the end ofstep (d).

In some embodiments of either the first or second aspect of theinvention, a pattern can be embossed into the first solid skin of thearticle during the method, and/or an external sleeve (which may bedecorative) can be formed on the outer surface, i.e. the first solidskin, of the article during the method. The pattern and/or decorationmay be, for example, a corporate logo. In such embodiments, step (e)further comprises, after removal of the outer part of the mould,inserting the article into a second mould before expansion of theplastic composition between the first and second skins (in said at leastone first portion, in the second aspect) is complete, and retaining thearticle in the second mould until the plastic composition between thefirst and second skins has solidified (in said at least one firstportion, in the second aspect). In the first aspect, when expansion ofthe plastic composition between the first and second skins is complete,the first solid skin of the article is in contact with a surface of thesecond mould. In the second aspect, when expansion of the plasticcomposition between the first and second skins in said at least onefirst portion is complete, the first solid skin of the article is incontact with a surface of the second mould in said at least one firstportion.

When it is desired to emboss a three-dimensional pattern into the outersurface of the article, the surface of the second mould may comprise athree-dimensional pattern. This pattern is then imparted to the firstsolid skin when it is forced into contact with the surface of the secondmould upon expansion of the plastic composition between the first andsecond solid skins. When the plastic composition comprisespolypropylene, the second mould would typically be heated to atemperature within a range of from 80 to 150° C. to deform the outersolid skin by embossing.

When it is desired to include a label on the outer surface of thearticle, then at least part of the surface of the second mould may becovered with a film sheet prior to inserting the article into the secondmould. If it is desired to cover the outer surface of the article with asleeve, then the film sheet may cover an entire circumference of atleast part of the surface of the second mould. For example, in thefinished article, the sleeve could cover an entire circumference of thearticle over only a part of the height of the article (e.g. a wide bandof the sleeve material around the mid-point of the article's height), orcould cover an entire circumference of the article over the entireheight of the article (with the exception of a rim area). In the firstaspect of the invention, at least part of the first solid skin of thearticle will be forced into contact with the film sheet upon expansionof the plastic composition between the first and second solid skins. Inthe second aspect of the invention, at least part of the first solidskin of the article will be forced into contact with the film sheet insaid at least one first portion upon expansion of the plasticcomposition between the first and second solid skins. The first solidskin may also be in contact with the film sheet in said at least onesecond portion. In both aspects, the outer surface (first solid skin) ofthe article bonds to the film sheet upon being forced into contacttherewith by the expansion of the plastic composition between the firstand second skins of the article. Again, the surface of the second mouldcould include a three-dimensional pattern, as above. The film sheetcould comprise plastics and/or paper. However, polypropylene (e.g.oriented polypropylene) is preferred as it is compatible with recycling.A foamed plastic (e.g. polypropylene) could also be used as the filmsheet, in order to provide added insulation to the article and toprovide a more tactile surface for gripping of the article. A label maytypically be placed into the mould cavity and forced into thecorrugations together with the injected plastic, and the label wouldthen follow the contour of the outer skin.

In those embodiments wherein the article is inserted into a second mouldbefore expansion of the plastic composition between the first and secondskins is complete, the second skin of the article remains in contactwith the second cavity-forming surface of the original mould, and thesecond mould is moved so as to be placed over the first skin of thearticle. The second mould could be moved by way of a robotic device e.g.a robotic arm. When expansion of the plastic composition between thefirst and second skins is complete and the first solid skin is incontact with the surface of the second mould (or the film sheet whichcovers at least part of the surface of the second mould), the articlemay then be released from the second cavity-forming surface of theoriginal mould and retained in the second mould. The second mould maythen be moved by the robotic device to take the articles for stacking.

In any embodiments of either of the above aspects of the presentinvention, the polymer of the plastic composition may comprise apolyolefin or blend of a plurality of polyolefins, optionallypolyethylene or polypropylene; or a polyester, optionally polyethyleneterephthalate or polybutylene terephthalate; or polylactic acid. In oneembodiment, the polymer comprises polypropylene. Polypropylenes having aMelt Flow Index (MFI) of from 10 to 120 are particularly preferred. TheMelt Flow Index of a polymer can be measured according to ASTM D1238.

Blowing agents which can be used in any embodiments of either of theabove aspects of the present invention include chemical blowing agents(which decompose under the conditions of the methods, thus liberatinggases into the molten plastic composition, which gases can subsequentlyexpand upon release of pressure on the plastic composition, e.g. openingthe mould, causing the plastic composition to foam and expand). Examplesof such chemical blowing agents include Hydrocerol® (from Clariant) andTecoCell® (from Trexel). The blowing agent is typically endothermic, butexothermic blowing agents could also be used. Alternatively, the blowingagent could be a physical blowing agent in the form of a gas dissolvedin the molten plastic composition. Such a gas may comprise, for example,carbon dioxide or nitrogen. The gas may optionally further include aperfume composition (i.e. a scent) which remains present in the polymermaterial after expansion, to enhance the consumer experience. Theblowing agent may be a chemical blowing agent or a physical blowingagent or any mixture of chemical and/or physical blowing agents. Forexample, the blowing agent may be a mixture of chemical blowing agents,for example a combination of endothermic and exothermic chemical blowingagents, or a combination of chemical and physical blowing agents, forexample where the chemical blowing agent helps to nucleate a gas formingthe physical blowing agent. Alternatively, the blowing agent may be amixture of physical gases, for example a mixture of CO₂ and N₂,optionally in further combination with a chemical blowing agent.

When using carbon dioxide as the blowing agent, CO₂ gas is produced bythe blowing agent in the extruder of the injection moulding machine, andthe CO₂ gas then goes into solution during the injection phase(typically from 300 to 500 bar) due to the relatively high pressureexerted on the material being greater than the pressure required(typically less than 100 bar) to force CO₂ into solution within moltenthermoplastic resin, such as polypropylene. When nitrogen is used as theblowing agent, the nitrogen does not go into solution within moltenpolypropylene, since nitrogen gas requires over 3,000 bar to force itinto partial solution into molten polypropylene.

In some embodiments of the first and second aspects of the presentinvention, the molten plastic composition includes a filler. Fillers mayact to add strength to the article, increase its thermal conductivity,or raise the heat distortion temperature of the article. In certainembodiments, the filler is chalk or calcium carbonate.

In both the first and second aspects of the present invention, thearticle may be a cup or container, e.g. a coffee cup or a containersuitable for warming soup in a microwave. The articles may bedisposable.

A further embodiment of a hollow container produced according to thepresent invention is further described below with reference to FIGS. 9to 14. In this embodiment, a coffee cup, i.e. a cup suitable for holdinghot beverages such as coffee, was manufactured according to the presentinvention. In this embodiment, the coffee cup has a capacity of 16 fluidounces. However, the hollow container may be used, or shaped anddimensioned fur use, as a container for food.

Initially in accordance with the present invention, a cup-like preform200, as shown in FIGS. 9 to 11, is injection moulded from athermoplastic resin comprising a blowing agent, in this examplepolypropylene and a carbon dioxide (CO₂) blowing agent. In the preform200, the polypropylene includes non-foamed regions in which carbondioxide (CO₂) gas is in solution in the polypropylene after theinjection moulding process and expanded cellular foamed regions in whichthe carbon dioxide (CO₂) gas has come out of solution in thepolypropylene during the injection moulding process, and has therebyformed cellular foam regions. However, any other suitable blowing agentand thermoplastic resin combination may be employed. The blowing agentforms foamed areas in the preform as a result of localised expansion ofthe thermoplastic resin in a low pressure region of the injection mould.

The cup-like preform 200 is injection moulded so as to have an annularfoot 202 which comprises unfoamed thermoplastic resin. Above the annularfoot 202 is a circular base wall 204, shown in detail in FIG. 11.

The annular sidewall 206 extends upwardly away from the annular foot 202and circular base wall 204 to terminate in an annular top rim 208.

The cup-like preform 200 had a height of 135 mm, a top rim diameter of90 mm and a base diameter of 55 mm The annular foot 202 had a height ofabout 2 mm The thermoplastic material of the cup-like preform 200 had anunexpanded volume (i.e. the material volume of the injection-mouldedpreform prior to further expansion to form the final cup) of 26 cm³. Thethermoplastic material of the cup-like preform 200 had an averagemoulded density of 0.9 g/cm³ and a weight of 24 grams. The annularsidewall 206 had an average length L/thickness T ratio of 180:1, where Lis the length of the sidewall 206 along the height of the sidewall 206and T is the sidewall thickness. Lower L/T ratios of less than 180:1 canbe used. However, lower L/T ratios imply a higher wall thickness for agiven cup height and tend to increase the weight of the cup and reducethe filling pressure, allowing the blowing agent to come out of solutionduring injection.

As shown in FIG. 9, the annular sidewall 206 comprises a plurality oflongitudinal ribs 210 extending radially outwardly from the outersurface 211 of the annular sidewall 206. The longitudinal ribs 210comprise alternating thick, or major, ribs 212 and thin, or minor, ribs214. FIG. 10 is an enlarged cross-section through the annular sidewall206. The thick ribs 212 and thin ribs 214 comprises cellular foamedthermoplastic material, and between the longitudinal ribs 210 arevalleys 216 which comprise unfoamed thermoplastic material. The valleys216 have a width, in a circumferential direction around the annularsidewall 206, from 0.3 to 1.0 mm in order to provide an unexpandedregion, i.e. the valleys 216, between expanded regions, i.e. the ribs210. The valleys 216 of unfoamed thermoplastic material typically have awidth, in a circumferential direction around the annular sidewall 206,of about 1 mm.

During the injection moulding process, in the locations of thelongitudinal ribs 210 the thermoplastic material has been subjected areduced pressure, as a result of the correspondingly thick regions ofthe moulding cavity, which has enabled the blowing agent to come out ofsolution and form a gas so as to form the expanded cellular foam withinthe injection mould. In contrast, during the injection moulding process,in the locations of the valleys 216 between the longitudinal ribs 210the thermoplastic material has been subjected a high pressure, as aresult of the correspondingly thin regions of the moulding cavity, whichhas prevented the blowing agent from coming out of solution, therebypreventing expansion of the thermoplastic material within the injectionmould.

Similarly, the annular top rim 208 is formed of expanded cellular foamwithin the injection mould and an annular transition zone 218 betweenthe annular top rim 208 and the upper ends of the longitudinal ribs 210and valleys 216 is formed of unexpanded thermoplastic material, sincethe annular top rim 208 is relatively thick whereas the annulartransition zone 218 is relatively thin. In addition, the relatively thinannular foot 202 is formed of unexpanded thermoplastic material.

FIG. 11 shows the circular base wall 204. The base wall 204 has acentral gate 220 which comprises the injection point for the injectionmoulding. The central gate 220 is surrounded by a first annular thicksection 222. A plurality of primary flow leaders 224, in the illustratedembodiment there are five primary flow leaders 224 but any suitablenumber may be provided, extend radially outwardly from the first annularthick section 222. The primary flow leaders 224 terminate at a secondannular thick section 226. A plurality of secondary flow leaders 228, inthe illustrated embodiment there are forty secondary flow leaders 228but any suitable number may be provided, extend radially outwardly fromthe second annular thick section 226. The secondary flow leaders 228each terminate at a lower end of a respective thin rib 214. The numberof secondary flow leaders 228 corresponds to the number of thin ribs214, and also corresponds to the number of thick ribs 212 which arerespectively alternately located between adjacent thin ribs 214.

The first annular thick section 222, the primary flow leaders 224, thesecond annular thick section 226 and the secondary flow leaders 228 havea thickness such that, as explained above for the longitudinal ribs 210,these elements are composed of expanded cellular foam within theinjection mould. Typically, these elements have a thickness of from 0.5to 1.0 mm, for example about 0.6 mm In contrast, there are firstsegments 230 between the primary flow leaders 224 and second segments232 between the secondary flow leaders 228 which have a thickness suchthat, as explained above for the valleys 216, these elements arecomposed of unexpanded thermoplastic material. Typically, these elementshave a thickness of from 0.2 to less than 0.5 mm, for example about 0.3mm.

The dimensions, namely the thickness and width (respectivelyperpendicular and parallel to the plane of the circular base wall 204)of the first annular thick section 222, the primary flow leaders 224,the second annular thick section 226 and the secondary flow leaders 228are selected so that during the injection moulding the thermoplasticmaterial can readily flow away from the central gate 220 to permit easymaterial flow to reduce the filling pressure and assist fast injection.The thickness of the first annular thick section 222, the primary flowleaders 224, the second annular thick section 226 and the secondary flowleaders 228 is set by the injection moulding and the resultant expandedcellular foam has a thickness which is less than the height (typically 2mm) of the annular foot 202. This ensures that the resultant cup can bestood securely on a flat surface around the circumference of the annularfoot 202 and the expanded cellular foam regions in the circular basewall 204 do not extend below the bottom edge of the annular foot 202 andthereby impede the cup from standing securely and reliably on a flatsurface.

In the photographs of FIGS. 9 to 11, the unexpanded areas appeartranslucent to the naked eye, since the blowing agent, in thisembodiment CO₂ gas, stays in solution. However, if a pigment isincorporated into the thermoplastic material, the unexpanded areastypically appear opaque, with a solid colour. The foamed areas typicallyappear to have a pastel colour due to the white background created bythe expanded cellular foam. In FIGS. 9 to 11, it should be noted thatcell sizes of less than 0.5 microns are not visible to the naked eye.

As described hereinabove, the injection moulding forms outer and innersolid skins 240, 242 at the outer and inner surfaces 244, 246 of thepreform 200, but the cooling time within the mould is minimized so thata molten thermoplastic resin is retained between the outer and innersolid skins 240, 242. After the injection moulding, the mould is openedand the preform 200 is removed from the outer moulding element, asdescribed hereinabove, before the thermoplastic resin between the innerand outer skins 240, 242 has solidified. Removal of the preform 200 fromthe outer moulding element reduces the pressure on the outer surface 244of the preform 200, which permits the blowing agent in the unexpandedregions of the thermoplastic material to come out of solution and form agas so as to form the expanded cellular foam externally of the injectionmould.

In an alternative embodiment, all of the longitudinal ribs 210 have thesame dimensions. In further alternative embodiments, the longitudinalribs 210 and valleys 216 can have independently varying dimensions.

The resultant cup structure is shown in FIGS. 12 to 14. The cup 250 is ahollow article for use as a beverage cup and has an annular sidewall 256and a base wall 204 defining a central hollow cavity 270. The cup 250has a top rim 208, comprises of expanded cellular foam, of the sidewall256 and a bottom end 272 of the sidewall 256. The sidewall 256 is anintegral annular moulding, and most preferably the annular sidewall2567, base wall 204, top rim 208 and bottom end 272 are an integralannular moulding. The hollow article is composed of a single plasticsmaterial, optionally a thermoplastic. Typically, the hollow article is abeverage cup or food container composed of a single recyclablethermoplastic material, optionally wherein the thermoplastic material isa polymer which comprises a polyolefin or blend of a plurality ofpolyolefins, further optionally polyethylene or polypropylene; or apolyester, further optionally polyethylene terephthalate or polybutyleneterephthalate; or polylactic acid. Preferably, the hollow article is acoffee cup which is thermally stable up to a temperature of at least 75°C.

The annular sidewall 256 comprises a plastics material composed of asandwich structure of inner and outer skins 254, 252 and an expandedcellular foam layer 257 therebetween. The top rim 208 is separated froman upper edge of the sandwich structure by an annular ring 218 of theplastics material which is unexpanded. The expanded cellular foam layer257 comprises an annular array of reinforcing areas 258 extendinglongitudinally along the sidewall 256 in a direction between the top rim208 and the bottom end 272. The reinforcing areas 258 are mutuallyseparated by an annular array of spacer regions 260 extendinglongitudinally along the sidewall 256 in a direction between the top rim208 and the bottom end 272 to provide alternating reinforcing areas 258and spacer regions 260 around the annular sidewall 256. The reinforcingareas 258 comprise expanded cellular foam of a first density and thespacer regions 260 comprise expanded cellular foam of a second density,wherein the first density is higher than the second density. Theexpanded cellular foam in the reinforcing areas 258 typically has alower concentration of cellular voids than the expanded cellular foam inthe spacer regions 260; and/or a more uniform size distribution ofcellular voids than the expanded cellular foam in the spacer regions260; and/or a smaller average size of cellular voids than the expandedcellular foam in the spacer regions 260.

In the resultant cup 250, the low-density expanded foam has remained inthe major and minor ribs 212, 214 which were present in the preform 200.The valleys 216 of the preform 200 are expanded from the valley floor,defined by the outer solid skin 240 of the valley 216, to a point higherthan the adjacent major and minor ribs 212, 214. However, surprisingly ahigher density foam is created between the major and minor ribs 212,which is believed to have a high density because no foam expansionduring preforming has taken place in the valleys 216.

In the circular base wall 204 of the preform, the structure issubstantially retained in the final cup base, although there may be someminor expansion of the previously unexpanded regions. The annulartransition zone 218 between the annular top rim 208 and the upper endsof the longitudinal ribs 210 and valleys 216 remains composed ofunexpanded thermoplastic material in the cup 250.

As shown in particular in FIG. 13, the resultant cup has a slightlyundulating outer circumferential surface 252 and substantially smoothinner circumferential surface 254 for the sidewall 256. The sidewall 256comprises expanded cellular foam 257. The outer circumferential surface252 has slightly higher surfaces at the location of the valleys 216 ofthe preform 200. However, optimization of the blowing agentconcentration, cooling time and preform temperature on removal from theinjection mould can be manipulated to achieve a substantially smoothouter circumferential surface 252.

As also described above with reference to FIG. 5, the foam densityvaries around the circumference of the sidewall 256, alternating betweenrelatively high density areas 258, constituting the reinforcing areas258, corresponding to the location of the valleys 216 of the preform200, and relatively low density areas 260, constituting the spacerregions 260, corresponding to the location of the longitudinal ribs 210of the preform 200. The alternating reinforcing areas 258 and spacerregions 260 around the annular sidewall 256 provide that eachreinforcing area 258 is located between opposite spacer regions 260 andeach spacer region 260 is located between opposite reinforcing areas258.

The low density areas 260 comprise first low density areas 260 acorresponding to the location of the major longitudinal ribs 212 of thepreform 200 and second low density areas 260 b corresponding to thelocation of the minor longitudinal ribs 214 of the preform 200, with thefoam density being slightly higher in the second low density areas 260 bthan in the first low density areas 260 a, but in each case the foamdensity is lower than in the relatively high density areas 258,corresponding to the location of the valleys 216. In the alternativeembodiment where the longitudinal ribs 210 have the same dimensions, thelow density areas 260 have the same dimensions and properties andalternate with the high density areas 258.

The spacer regions 260 comprise first and second spacer regions 260 a,260 b, the first spacer regions 260 a having a greater width, in acircumferential direction around the annular sidewall 256, than thesecond spacer regions 260 b. The first and second spacer regions 260 a,260 b comprise expanded cellular foam, and the expanded cellular foam ofthe first spacer regions 260 a has a lower density than the expandedcellular foam of the second spacer regions 260 b. The first and secondspacer regions 260 a, 260 b alternate around the annular sidewall 256.The alternating reinforcing areas 258 and first and second spacerregions 260 a, 260 b around the annular sidewall provide a repeatingsequence of a reinforcing area 258, a first spacer region 260 a, areinforcing area 258, and a second spacer region 260 b.

The reinforcing areas 258 have a width, in a circumferential directionaround the annular sidewall, of from 0.5 to 3 mm, optionally from 0.75to 2 mm. The spacer regions 260 have a width, in a circumferentialdirection around the annular sidewall, of from 0.5 to 10 mm optionallyfrom 0.5 to 4 mm, further optionally from 0.75 to 3 mm The sidewall hasa thickness of from 0.5 to 4 mm, optionally from 1 to 3 mm.

As described above, in the circular base wall 204 of the preform, thestructure is substantially retained in the final cup base, althoughthere may be some minor expansion of the previously unexpanded regions.Therefore in the hollow article or cup 250, the base wall 204 comprisesa central gate region 280, a first annular ring 282 of expanded cellularfoam surrounding the gate region 280, and a plurality of first radialelements 284 of expanded cellular foam extending radially outwardlytowards the sidewall 256. The first radial elements 284 are mutuallyseparated by first segments 286 of unexpanded plastics material. Thebase wall 204 further comprises a second annular ring 288 of expandedcellular foam surrounding, and connecting with the radially outer ends290 of, the plurality of first radial elements 284, and a plurality ofsecond radial elements 292 of expanded cellular foam extending radiallyoutwardly towards the sidewall 256. The second radial elements 292 aremutually separated by second segments 294 of unexpanded plasticsmaterial. The radially outer ends 296 of the plurality of second radialelements 294 of expanded cellular foam connect with the sidewall 256, inparticular connect with a respective spacer region 260, typically thefirst spacer region 260 a, in the sidewall 256.

The unexpanded plastics material in the base wall 204 typically has athickness of from 0.25 to 0.75 mm, optionally from 0.25 to 0.5 mm Theexpanded cellular foam of plastics material in the base wall 204typically has a thickness of from 0.5 to 1.75 mm, optionally from 0.5 to1.25 mm.

The hollow article 250 further comprises at least one foot 298 whichextends downwardly from the bottom end 272 of the sidewall 256 anddefines at least one lower surface 300 which is located lower than abottom surface 302 of the base wall 204. Preferably, the at least onefoot 298 comprises a single annular foot 298 having a single annularlower surface 300 and comprises unexpanded plastics material which isintegrally moulded with the sidewall 256 and the base wall 204 and has aheight of from 1.5 to 4 mm, optionally from 1.75 to 3 mm

The alternating and repeating foam density around the cup 250 is shownin FIGS. 13 and 14. The resulting foam structure, of alternatinghigh/low density expanded cellular foam areas around the cupcircumference, provides a high load capacity of the cup when loaded in alongitudinal direction; in other words, the cup has a high resistance tobeing crushed by a force applied along the longitudinal axis of the cup.The alternating high/low density expanded cellular foam areas extendlongitudinally along the sidewall, so that the high density areas 258constitute longitudinal reinforcing ribs separated by longitudinal lowerdensity foam areas 260 a, 260 b. This longitudinal crush strength issubstantially doubled as compared to a foam side wall of a correspondingthickness but with a constant foam density around the cup circumference.The cup also has very high flexural stiffness and very high hoopstiffness in the sidewall, which is achieved with a significantlyreduced mass of thermoplastic material as compared to cup sidewalls ofunfoamed thermoplastic material of corresponding structural properties.In short, the foamed cup of the present invention can provide highstructural strength to a sidewall using a minimum amount ofthermoplastic material in a cup which can be formed from a singlethermoplastic material.

The rigidity of the cup formed using the preferred embodiments of thepresent invention is far superior to any other limited-use cup currentlyin the market. The cup typically comprises a thermoplastic polymer whichcan have high crystallinity in both the foamed and unfoamed portions,and therefore has high thermal stability. The preferred cup of thepresent invention is therefore dishwasher-safe and microwave-safe, andcan exhibit unlimited reuse.

The present invention can provide a high strength hollow article with ahigh ratio of volume to weight. For example, a ratio between the volumeof the central hollow cavity, in cm³, and the mass of the hollowarticle, in g, is from 2 to 3.

The thermoplastic material of the cup 250 of the illustrated embodimenthas an expanded volume (i.e. the material volume of the final cup) of55.6 cm³ and, compared to the preform which has an unexpanded volume of26 cm³, this represents an expansion of about 110% between the cup andthe preform which occurs outside the injection mould as a result ofsignificant expansion circumferentially outwardly of the valleys 216,and reduced expansion circumferentially outwardly of the longitudinalribs 210 to form the foam sidewall 256 of the cup 250.

Typically, the volumetric change from the intermediate preformed cup tothe final fully expanded cup is about 2.1:1, but this ratio can readilybe varied by design and process control to be within the range of from1.5:1 to 3:1.

In the illustrated embodiment of FIGS. 9 to 14 a typical coffee cup isformed. However, the present invention may be employed to produce ahollow container having a height as low as about 10 mm, for exampleforming a tray, or a wide-mouth container such as a tub, for example atub for hot or cold foodstuffs, e.g. takeaway food, cook-chill food orready to eat food.

Various modifications to the illustrated embodiments will be apparent tothose skilled in the art and are intended to be included within thescope of the present invention.

1.-45. (canceled)
 46. A hollow article for use as a beverage cup, or asa container, the hollow article having an annular sidewall and a basewall defining a central hollow cavity, a top rim of the sidewall and abottom end of the sidewall, wherein the annular sidewall comprises aplastics material composed of a sandwich structure of inner and outerskins and an expanded cellular foam layer therebetween, wherein theexpanded cellular foam layer comprises an annular array of reinforcingareas extending longitudinally along the sidewall in a direction betweenthe top rim and the bottom end, the reinforcing areas being mutuallyseparated by an annular array of spacer regions extending longitudinallyalong the sidewall in a direction between the top rim and the bottom endto provide alternating reinforcing areas and spacer regions around theannular sidewall, wherein the reinforcing areas comprise expandedcellular foam of a first density and the spacer regions compriseexpanded cellular foam of a second density, wherein the first density ishigher than the second density, wherein the expanded cellular foam inthe reinforcing areas of the first density has a more uniform sizedistribution, and a smaller average size, of cellular voids than theexpanded cellular foam of the second density in the spacer regions. 47.A hollow article according to claim 46, wherein the expanded cellularfoam in the reinforcing areas has a lower concentration of cellularvoids than the expanded cellular foam in the spacer regions. 48.(canceled)
 49. (canceled)
 50. A hollow article according to claim 46,wherein the alternating reinforcing areas and spacer regions around theannular sidewall provide that each reinforcing area is located betweenopposite spacer regions and each spacer region is located betweenopposite reinforcing areas.
 51. A hollow article according to claim 46,wherein the spacer regions comprise first and second spacer regions, thefirst spacer regions having a greater width, in a circumferentialdirection around the annular sidewall, than the second spacer regions,wherein the first and second spacer regions comprise expanded cellularfoam, and the expanded cellular foam of the first spacer regions has alower density than the expanded cellular foam of the second spacerregions.
 52. (canceled)
 53. A hollow article according to claim 51,wherein the first and second spacer regions alternate around the annularsidewall, and wherein the alternating reinforcing areas and first andsecond spacer regions around the annular sidewall provide a repeatingsequence of a reinforcing area, a first spacer region, a reinforcingarea, and a second spacer region.
 54. (canceled)
 55. A hollow articleaccording to claim 46, wherein the reinforcing areas have a width, in acircumferential direction around the annular sidewall, of from 0.5 to 3mm, or from 0.75 to 2 mm, and wherein the spacer regions have a width,in a circumferential direction around the annular sidewall, of from 0.5to 10 mm, from 0.5 to 4 mm, or from 0.75 to 3 mm.
 56. (canceled)
 57. Ahollow article according to claim 46, wherein the sidewall has athickness of from 0.5 to 4 mm, or from 1 to 3 mm.
 58. (canceled)
 59. Ahollow article according to claim 46, wherein the annular sidewall, basewall, top rim and bottom end are an integral annular moulding and thehollow article is composed of a single thermoplastic material. 60.(canceled)
 61. A hollow article according to claim 46, wherein the toprim comprises an annular ring of expanded cellular foam, and wherein thetop rim is separated from an upper edge of the sandwich structure by anannular ring of the plastics material which is unexpanded. 62.(canceled)
 63. A hollow article according to claim 46, wherein the basewall comprises a central gate region, a first annular ring of expandedcellular foam surrounding the gate region, and a plurality of firstradial elements of expanded cellular foam extending radially outwardlytowards the sidewall, the first radial elements being mutually separatedby first segments of unexpanded plastics material.
 64. A hollow articleaccording to claim 63, wherein the base wall further comprises a secondannular ring of expanded cellular foam surrounding, and connecting withthe radially outer ends of, the plurality of first radial elements, anda plurality of second radial elements of expanded cellular foamextending radially outwardly towards the sidewall, the second radialelements being mutually separated by second segments of unexpandedplastics material, the radially outer ends of the plurality of secondradial elements of expanded cellular foam connecting with the sidewall.65. A hollow article according to claim 64, wherein the radially outerend of each of the plurality of second radial elements of expandedcellular foam connects with a respective spacer region in the sidewall.66. A hollow article according to claim 63, wherein the unexpandedplastics material in the base wall has a thickness of from 0.25 to 0.75mm, or from 0.25 to 0.5 mm.
 67. A hollow article according to claim 63,wherein the expanded cellular foam of plastics material in the base wallhas a thickness of from 0.5 to 1.75 mm, from 0.5 to 1.25 mm.
 68. Ahollow article according to claim 46, further comprising at least onefoot which extends downwardly from the bottom end of the sidewall anddefines at least one lower surface which is located lower than a bottomsurface of the base wall wherein the at least one foot is integrallymoulded with the sidewall and the base wall.
 69. A hollow articleaccording to claim 68, wherein the at least one foot comprises a singleannular foot having a single annular lower surface and the at least onefoot comprises unexpanded plastics material.
 70. (canceled) 71.(canceled)
 72. A hollow article according to claim 68, wherein the atleast one foot has a height of from 1.5 to 4 mm, or from 1.75 to 3 mm.73. A hollow article according to claim 46, which is a beverage cup orfood container composed of a single recyclable thermoplastic material,wherein the thermoplastic material is polyethylene, polypropylene,polyethylene terephthalate, polybutylene terephthalate, or polylacticacid.
 74. A hollow article according to claim 46, which is a coffee cupwhich is thermally stable up to a temperature of at least 75° C.
 75. Ahollow article according to claim 46, wherein a ratio between the volumeof the central hollow cavity, in cm³, and the mass of the hollowarticle, in g, is from 2 to
 3. 76. A hollow article according to claim46, wherein the annular sidewall has a substantially smooth innercylindrical surface and an outer surface which is substantially smoothor is slightly undulating with slightly higher surfaces at thereinforcing areas.